(1) Field of the Invention
The present invention relates to a novel method of colorimetrically measuring zinc.
(2) Description of the Prior Art
Recently, the quantitative measurement of a small amount of a metal rises in importance with respect to environmental sanitation and biochemical diagnosis method. In particular, zinc is a metal present in a small amount in the living organisms and one of the indispensable metal elements necessary for the growth, and is widely distributed in the bodies of the living organisms. It has been lately reported that the concentration of zinc in serum lowers in positive acrodermatitis, taste impediment, SLE (systemic lupus erythematosus), zinc deficiency symptom observed in the case of administration of a high calorie transfusion, and so on, and the necessity of the measurement of the zinc in serum has been felt in the diagnosis, treatment and prognosis of these diseases.
As the method of quantitatively measuring a small amount of zinc until today, the atomic absorption method, the flame photometric method, and the colorimetrically measuring method are mainly used, and the colorimetrically measuring method is advantageous from the standpoint that a number of specimens to be examined can be promptly treated and no special analysing device is necessary.
As the colorimetric reagent, use may be mainly made of dithizone, zincon, PAN{1-(2-pyridylazo)-2-naphthol}, but any one of them is not necessarily complete from the viewpoint of specificity. In order to remove the interference of iron, copper, cobalt and manganese, it was necessary to use a cyano compound or a variety of masking agents in combination or use an extraction with an organic solvent.
Under the circumstances, the present inventors have noted the compounds disclosed in Japanese Patent Application Laid-Open No. 91,975,1982 which have been developed as a high sensitivity colorimetrically measuring reagents for a small amount of a metal and have a molecular absorption coefficient of 7.0-13.3.times.10.sup.-4. Although these compounds are all useful as the high sensitivity colorimetrically measuring reagents for the heavy metal, they have the defect that they sense a small amount of a metal other than zinc in the case of pyridylazophenol compounds like in the pyridylazonaphtol compounds because their general non-specificity.
Accordingly, when the above compounds are used, metals other than the intended specific ones are required to be masked as conventionally done.
For this purpose, use have generally been made of citrates, condensed phosphate, sodium fluoride, and nitrilotriacetic acid, 1-hydroxyethane-1,1-diphosphonic acid for the masking agents of iron; salicylaldoxime, 2-mercaptobenzothiazol, dithiocarboxyzarcosine, and dithiocarboxy glycine for the masking agents of copper; and dithiocarboxyzarcosine, and 2-mercaptobenzothiazol for the masking agents of cobalt.
Thus, they are very inconvenient in that some of these masking agents mask the intended zinc to lower the sensitivity thereof (for instance, citric acid, condensed phosphoric acid), and substances containing a partial structure --S--C.dbd.S or and --SH group, which are used as the masking agents of copper and cobalt, are conspicuously poor in stability after the preparation thereof, and these substances may be a coloring promotor of iron because they reduce trivalent iron to divalent iron.